Photocatalyst

ABSTRACT

A Ti—O—N film is formed on an SiO 2  substrate by sputtering. For example, TiO 2  is used as a target and nitrogen gas is introduced into the atmosphere. Crystallization is carried out by a post-sputtering heat treatment. Then a charge separation material such as Pt is supported on the Ti—O—N film. With the fabricated TiO 2  crystals, the Ti—O—N film containing nitrogen exhibits a good catalytic reaction by using visible light as acting light. Since the charge separation material captures electrons or positive holes, recombination of electrons and positive holes is effectively prevented, and consequently more efficient photocatalytic reaction is performed. It is preferable to form a photocatalyst material film (Ti—Cr—O—N film) by sputtering the SiO 2  substrate by use of TiO 2  and Cr as the target in a nitrogen atmosphere. Crystallization is performed by a post-sputtering heat treatment.

TECHNICAL FIELD

The present invention relates to a photocatalyst that can produceeffects when exposed to visible light.

BACKGROUND ART

Conventionally, various materials such as, for example, TiO₂ (titaniumdioxide), CdS (cadmium sulfide), WO₃ (tungsten trioxide), and ZnO (zincoxide) are known as materials for producing photocatalytic effects.These photocatalytic materials are semiconductors which absorb light toproduce electrons and holes, and cause various chemical reactions anddisinfection effects. Currently, the only material put in practice isTiO₂, because TiO₂ is superior in consideration of toxicity andstability with respect to exposure to water, acids, and bases.

However, because of the band gap value of TiO₂ (Eg=3.2 eV in an anatasecrystal), the operational light of the TiO₂ photocatalyst is limited toultraviolet light having a wavelength of less than 380 nm. In order toallow satisfactory operation under sunlight, indoors, or in a vehicle,and to improvement catalytic activity when light of weak intensity isirradiated, there is strong demand for development of a material whichcan realize catalytic activity when irradiated by visible light having awavelength longer than or equal to 380 nm.

For example, Japanese Patent Laid-Open Publication No. Hei 9-262482discloses modification of material through ion implantation of a metalelement such as, for example, Cr (chromium), and V (vanadium) to ananatase TiO₂ which has a high catalytic activity, in order to shift thelight absorption edge of TiO₂ towards a longer wavelength and to therebyenable operation of a TiO₂ catalyst under visible light. Although dopingof Cr, V, or the like has been reported since the early 1970's, none ofthe early reports disclose that operation by visible light is enabled.In Japanese Patent Laid-Open Publication No. Hei 9-262482, operation byvisible light are enabled by using a special doping method, ionimplantation, for Cr, V, or the like.

In the above conventional art, operability under visible light of a TiO₂photocatalyst is enabled through ion implantation of a metal element toTiO₂. However, ion implantation of a metal element is likely to requirelarge and expensive apparatus. To this end, there is a demand forsynthesizing the TiO₂ photocatalyst through other methods, such as, forexample, synthesis in solution or sputtering. However, photocatalystcreated through these methods cannot operate by visible light. It isconsidered that this is because aggregation of the dopant, Cr, occurs orbecause oxides such as Cr₂O₃ are formed during the crystallizationprocesses. As described, in the conventional art, there has been aproblem that, in order to enable operation of TiO₂ by visible lightusing a metal element, ion implantation of the metal element isrequired.

DISCLOSURE OF INVENTION

One object of the present invention is to realize a TiO₂ photocatalystcapable of operating in the visible light in addition to the ultravioletrange by using a novel material and without using costly productionmethods such as ion implantation.

According to a first aspect of the present invention, there is provideda photocatalyst comprising, as an inner material, a titanium compound(Ti—O—N or Ti—O—S) in which a nitrogen atom (N) or a sulfur atom (S)substitutes for a portion of the oxygen site of crystals of titaniumoxide (for example, TiO₂), is doped at an interstitial site of thecrystal lattices of titanium oxide, or is placed at the grain boundaryof polycrystalline assembly of titanium oxide crystals, and wherein acharge separation material is partially supported on the surface of thetitanium compound.

Ti—O—N or Ti—O—S are titanium compounds obtained by introducing nitrogenor sulfur to titanium oxide crystals and have an active photocatalyticfunction not only when exposed to light in the ultraviolet range, butalso under light in the visible range. Therefore, the photocatalyticfunction similar to that in TiO₂ can be obtained with visible light asthe operational light.

Moreover, a charge separation material can be partially supported on thesurface of Ti—O—N or Ti—O—S. As the charge separation material, forexample, at least one of Pt, Pd, Ni, RuO_(x) (for example, RuO₂),NiO_(x) (for example, NiO), SnO_(x) (for example, SnO₂), Al_(x)O_(y)(for example, Al₂O₃), ZnO_(x) (for example, ZnO), and SiO_(x) (forexample, SiO₂) may be selected. Such a charge separation material actsas a promoter and facilitates separation of charges produced as a resultof irradiation of light. That is, a metal element such as Pt, Pd, and Niselectively captures electrons and an oxide such as RuO_(x) (forexample, RuO₂), NiO_(x) (for example, NiO), SnO_(x) (for example, SnO₂),Al_(x)O_(y) (for example, Al₂O₃), ZnO_(x) (for example, ZnO), andSiO_(x) (for example, SiO₂) selectively captures holes. Therefore, bypartially supporting these materials on the surface of thephotocatalytic material, the probability of recombination of electronsand holes produced by the photocatalytic reaction is reduced and, thus,reduction in activity caused by the recombination of electrons and holescan be prevented.

It is preferable that the ratio, X %, of number of atoms of N in Ti—O—Nbe 0<X<13. A similar ratio is preferable for the S in Ti—O—S. It is alsopreferable that, when the metal element or oxide is assumed to beuniformly supported, the amount of the metal element or oxide on thesurface which acts as a promoter corresponds to a thickness of 0.1angstrom (Å) to 10 Å. In the case of SiO_(x), it is preferable that thecorresponding amount be 10 Å to 500 Å. In reality, these promoters onthe surface forms an island-like structure and may not be presententirely over the Ti—O—N or Ti—O—S surface.

According to another aspect of the present invention, it is preferablethat the Ti—O—N or Ti—O—S is used as an inner material, a titanium oxidelayer is formed on the surface of the inner material, and a chargeseparation material is partially supported on the surface of thetitanium oxide layer.

By employing such a structure, it is possible for the inner Ti—O—N orTi—O—S to absorb light in the range from ultraviolet to visible whileallowing catalytic reaction by the titanium oxide on the surface and thecharge separation material partially supported thereon. Titanium oxideis inexpensive and stable, and an effective catalytic reaction can berealized while preventing recombination of electrons and holes by Pt,Pd, Ni, RuO_(x) (for example, RuO₂), NiO_(x) (for example, NiO), SnO_(x)(for example, SnO₂), Al_(x)O_(y) (for example, Al₂O₃), ZnO_(x) (forexample, ZnO), and SiO_(x) (for example, SiO₂).

According to another aspect of the present invention, there is provideda photocatalytic material comprising an oxide crystal of a metal elementM1, the oxide crystal having a photocatalytic function, in which anitrogen atom or a sulfur atom substitutes for a portion of the oxygensites of the oxide crystals, is doped at an interstitial site of thecrystal lattices of oxide, or is placed at the grain boundary of thepolycrystalline body of oxide crystals, and wherein at least one metalelement M2 of vanadium (V), chromium (Cr), manganese (Mn), iron (Fe),cobalt (Co), nickel (Ni), copper (Cu), zinc(Zn), ruthenium (Ru), rhodium(Rh), rhenium (Re), osmium (Os), palladium (Pd), platinum (Pt), iridium(Ir), niobium (Nb), and molybdenum (Mo) substitutes for a portion of theM1 sites of the oxide crystal, is doped at an interstitial site of thecrystal lattices of the oxide, or is placed at the grain boundary of thepolycrystalline body of the oxide crystals.

Here, it is preferable that the compositional ratio of nitrogen exceeds0 and is less than 13 in the ratio percent of number of atoms and thatthe compositional ratio of various metal elements exceeds 0 and is lessthan 5 in the ratio percent of number of atoms. The compositional ratioof sulfur is similar to that of nitrogen.

In such a photocatalytic material, the absorption edges in the lightabsorption spectrum are shifted towards a longer wavelength compared toTiO₂, Ti—O—N, and Ti—O—S. Therefore, light of a longer wavelength isabsorbed and generates a photocatalytic effect. As a result, theefficiency of photocatalytic functions, that is, characteristics such asdecomposition of organic matters, decomposition of poisonous gases,purification of water, or the like, can be improved for cases when thesunlight or fluorescent lamp is used as a light source. Moreover, thephotocatalytic material enables realization of hydrophiliccharacteristic and anti-fogging property on the surface not only byirradiation of ultraviolet ray, but also by irradiation of visiblelight, and maintains such characteristic for a longer period of time.

The cause of this can be considered as follows. The valence band of asemiconductor whose characteristics are dominated by oxygen, O, isaffected by doping of nitrogen, N, or sulfur, S. Similarly, conductionband characteristics dominated by Ti are affected by the doping ofmetals. As a result, one or more new energy levels are created withinthe band gap (forbidden band) of the oxide such as TiO₂, and theeffective band gap is narrowed. Consequently, electrons and holes can beproduced by absorbing light of lower energy and longer wavelength thanin the cases of TiO₂, Ti—O—N, and Ti—O—S.

According to another aspect of the present invention, it is preferablethat the metal element M1 be formed from any of titanium (Ti), zinc(Zn), and tin (Sn). The oxide of these metal elements M1 functions as aphotocatalyst, and the operational light is shifted towards longerwavelength by doping a metal element M2 as described above.

According to another aspect of the present invention, it is preferablethat the photocatalytic material is used as an inner material, and thata titanium oxide or a Ti—O—N layer or a Ti—O—S layer which is a titaniumoxide containing nitrogen or sulfur is formed as an outer material.

In this manner, by placing Ti—Cr—O—N or the like as an inner material,visible light of long wavelength can be effectively absorbed andelectrons and holes can be produced there. The electrons and holesmigrate to TiO₂, Ti—O—N, or Ti—O—S at the surface and superiorhydrophilicity, contamination prevention, or decomposition of organicmatters can be achieved on the surface. In addition, by placing morestable TiO₂, Ti—O—N or Ti—O—S on the front-most surface, the long-termstability of the structure can be improved compared to a structure ofsimply Ti—Cr—O—N or the like.

According to another aspect of the present invention, it is alsopreferable that the compositional ratios in the outer material and inthe inner material gradually change according to the distance from thesurface.

The photocatalyst according to this aspect of the present inventionbasically comprises a titanium compound (Ti—O—N or Ti—O—S) in which anitrogen atom (N) or a sulfur atom (S) substitutes for a portion of theoxygen sites of a metal oxide such as titanium oxide, is doped at aninterstitial site of lattices, or is placed at the grain boundary ofpolycrystalline body.

Such a metal oxide, for example, Ti—O—N or Ti—O—S in which nitrogen orsulfur is contained in titanium oxide crystals, demonstratesphotocatalytic effects when exposed to light in the visible andultraviolet ranges.

Furthermore, by further doping (co-doping) a metal as described above toTi—O—N or Ti—O—S, light at even longer wavelengths can also beeffectively absorbed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIGS. 2A and 2B are diagrams showing crystal lattices of TiO₂.

FIG. 3 is a diagram showing the photocatalytic functions of the firstembodiment.

FIG. 4 is a diagram showing an XPS spectrum of a Ti—O—N film.

FIGS. 5A and 5B are diagrams showing structures of a second embodimentof the present invention.

FIG. 6 is a diagram showing a structure of a third embodiment of thepresent invention.

FIG. 7 is a diagram showing a structure of a fourth embodiment of thepresent invention.

FIG. 8 is a diagram showing the density of states of Ti—M2—O.

FIG. 9 is a diagram showing the density of states of Ti—O—X.

FIG. 10 is a diagram showing light absorbance of Ti—O—M2—X.

FIG. 11 is a diagram showing photocatalytic functions of the fourthembodiment.

FIGS. 12A and 12B are diagrams showing a structure of a fifth embodimentof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiments of the present invention (hereinafter referredto as “embodiments”) will be described in the following while referringto the drawings.

(Embodiment 1)

FIG. 1 shows a structure of a first embodiment of the present invention.In this structure, a Ti—O—N film 12 which is a photocatalytic materialis formed on a substrate 10 and Pt and RuO₂ are partially deposited onthe surface of the Ti—O—N film 12. Various materials, such as glass andceramic, can be chosen for the substrate 10 according to the desiredapplication.

The Ti—O—N film 12 has a structure in which a nitrogen atom is dopedbetween lattices of TiO₂ crystals. The doping of nitrogen atoms can beachieved by any one or any combination of substituting a portion of theoxygen sites of the TiO₂ crystals by nitrogen, doping nitrogen atoms atinterstitial sites of the lattices of TiO₂ crystals, or placing nitrogenatoms at the grain boundary of polycrystalline assembly of TiO₂crystals.

The compositional ratio of each element in the Ti—O—N film 12 may be,for example, Ti₃₁O₆₇N₂. Therefore, the Ti—O—N film 12 has a structurewhich consists primarily of a crystal of TiO₂ and is doped with N. Thecrystal phase of the TiO₂ crystal may be either rutile, anatase, or acombination of anatase and rutile.

FIG. 2A shows a unit lattice of TiO₂ crystal of rutile phase and FIG. 2Bshows a unit lattice of TiO₂ crystal of anatase phase. In these figures,small circles represent Ti and large circles represent O. N substitutesa portion of O or is introduced at an interstitial site of the crystallattices or grain boundary of TiO₂ crystals so that Ti—O—N is formed.

In parallel to experiments performed by the present inventors, thepresent inventors have evaluated, through the full-potentiallinearized-augmented-plane-wave (FLAPW) calculation method, the electronstates and optical characteristics of Ti—O—X-based semiconductor photocatalyst in which an anion X is doped. As a result, it was found that N(nitrogen) and S (sulfur) are effective dopants X for allowing operationby visible light. From these results, it has been found that theadvantages of doping according to the present invention can be obtainednot only when oxygen (O) is substituted by another anion X, but alsowhen the anion X is present at an interstitial site of crystal latticein a form to deform the lattice or wherein the anion X is present in thecrystal grain boundary, or any combination of these cases, as long as aTi—X bond is present in the photocatalyst having titanium oxide as abase material. Similar advantages can also be obtained by anion dopingof amorphous titanium oxide.

Moreover, as long as N is doped in the manner described above, the ratioin the number of atoms between Ti and (O+N) need not be 1:2. Forexample, the compositional ratio may be Ti₃₁O₆₇N₂ as described above,which is excessive in oxygen, or Ti₃₇O₆₁N₂, which is somewhat reductive.This also applies to doping of S.

The photocatalytic material may be manufactured, for example, by RFmagnetron sputtering. An example manufacturing method will now bedescribed.

First, a substrate 10 and a TiO₂ target are set within a vacuum chamberof an RF magnetron sputtering apparatus. A predetermined amount of N₂gas and an inert gas (for example, Ar gas) is introduced into the vacuumchamber and sputtering is performed in (N₂+Ar) plasma. In this manner,Ti—O—N film 12 is deposited on the substrate 10.

The total gas pressure during the sputtering may be set at approximately0.52 Pa and the partial pressure of N₂ may be set such that 0%<(partialpressure of N₂)≦100%. It is preferable that the partial pressure of N₂be set approximately 20-60%, and be set at, for example, 40%. The inputpower for TiO₂ is, for example, 600 W ★2 using two targets.

After formation of the Ti—O—N film 12 by sputtering, thermal treatment(annealing) is applied for crystallization. For example, a thermaltreatment can be applied under a nitrogen atmosphere at 550° C. forapproximately 2 hours for crystallization. In other words, when the filmis deposited, the film has an amorphous+polycrystalline structure, butby applying a thermal treatment, polycrystallization and singlecrystallization can be achieved. It is also possible to deposit theTi—O—N film 12 while heating the substrate 10, and omit the thermaltreatment process after film deposition.

After the thermal treatment, Pt is deposited as a charge separationmaterial through sputtering. The Pt target may be set in the vacuumchamber in advance or at a later stage. The amount of deposition of Ptby sputtering is set to correspond to a thicknesses of 1 Å and 5 Å thatcan be obtained if Pt is assumed to be uniformly deposited over thesurface. In this manner, Pt islands are formed on the surface of theTi—O—N film 12. The charge separation material is not limited to Pt, andcan be any of Pd, Ni, RuO_(x) (for example, RuO₂), NiO_(x) (for example,NiO), SnO_(x) (for example, SnO₂), Al_(x)O_(y) (for example, Al₂O₃),ZnO_(x) (for example, ZnO), or SiO_(x) (for example, SiO₂), or anycombination of the above materials.

Using the above manufacturing method, a photocatalyst was produced in anexample by producing the Ti—O—N film 12 on the substrate 10 andpartially depositing Pt on the Ti—O—N film 12. Two photocatalysts wereproduced respectively having Pt in an amount corresponding to athickness of 1 Å and of 5 Å.

As comparative examples, a TiO₂ film, a Ti—O—N film, and structures inwhich Pt was partially deposited on TiO₂ films in an amountcorresponding to 1 Å and 5 Å were produced. The structures in which Ptwas partially deposited on TiO₂ films were produced as follows. First, aTiO₂ target was sputtered in a 20% O₂—Ar atmosphere, and then thestructure was annealed under O₂ atmosphere at 450° C. for 90 minutes forcrystallization. Pt was deposited on the surface in an amountcorresponding to thicknesses of 1 Å and 5 Å.

FIG. 3 shows a result of measurement of photocatalytic activities of the6 samples, TiO₂, 1 Å Pt/TiO₂, 5 Å Pt/TiO₂, Ti—O—N, 1 Å Pt/Ti—O—N, and 5Å Pt/Ti—O—N, formed through the above processes, the measurementrepresented by the decomposition performance of methylene blue. Morespecifically, methylene blue was applied to the surface of each film andthe decomposition performance involved with light irradiation wasmeasured as a change in absorbance of light having a wavelength of 600nm (ΔABS). An Xe lamp of 500 W was used as the radiation light source,and tests were performed for cases wherein light including anultraviolet component having wavelength λ of 200 nm or greater wasirradiated and wherein visible light having a wavelength λ of 380 nm orgreater was irradiated, wherein the wavelength of irradiated light waslimited using an optical filter.

From this result, it can be seen that the performance of Ti—O—N can besignificantly improved over that of TiO₂ because Ti—O—N exhibitsadditional photocatalytic reactivity in response to visible lightirradiation. It can also be seen that the performance as a photocatalystcan be further improved by a factor of approximately 2 by partiallydepositing Pt on Ti—O—N. The results for different amounts of Pt,corresponding to a 1 Å thickness and a 5 Å thickness, did not varysignificantly.

Ti—O—N will now be explained in more detail. Influences of the amount ofdoping of nitrogen to the TiO₂ crystal on the photocatalytic functionwere studied through additional experiments. For these experiments, thepercent ratio of number of nitrogen atoms in Ti—O—N film prepared underthe partial pressure of N₂ of 20% was 6.6% before the thermal treatmentand 1.4% after the thermal treatment. Similarly, the percent ratio ofnumber of nitrogen atoms in Ti—O—N film prepared under the partialpressure of N₂ of 100% was 12.7% before the thermal treatment and 0.5%after the thermal treatment. In Ti—O—N films prepared under the partialpressures of N₂ of 40% and 60%, the percent ratios of number of nitrogenatoms in the Ti—O—N film were respectively 1.4% and 1.5% after thethermal treatment. A photocatalytic function was observed in all of thetested Ti—O—N films. Therefore, the nitrogen content of the Ti—O—N film,when the percent ratio of number of atoms is represented by X %, ispreferably 0<X <13. The photocatalytic function of the Ti—O—N film issuperior in thermally treated films, and it is preferable that thenitrogen concentration after the thermal treatment be few % or less, andit is more preferable that the nitrogen concentration after the thermaltreatment be 2% of less.

In the production of the Ti—O—N film 12 as described above, a TiO₂target was used and the Ti—O—N film 12 was formed in a plasma of Ar gascontaining N₂. However, the Ti—O—N film 12 may also be formed using TiN(titanium nitride) target and in a plasma of gas containing O₂. It isalso possible to use a combination of TiO₂ and TiN as the target.Furthermore, it is also possible to form the Ti—O—N film 12 throughvacuum evaporation or ion plating in (N₂+O₂) gas using a Ti ingot.

In the example described above, the Ti—O—N photocatalytic material wasin the form of a thin film. However, the present invention may also beapplied to a structure having, as a base material, a structure wherein acharge separation material is partially deposited on the surface ofparticulate Ti—O—N which is mixed to a binder material for painting.

Further, Ti—O—N can also be created using the above method formanufacturing as a basis and through various particle creation methods,sol-gel methods, chemical reaction methods, or the like.

In particular, in the Ti—O—N film of the embodiment, a chemical bond ispresent between N and Ti. More specifically, the chemical bonding statesof nitrogen atom were determined from measurement results of a spectrumassociated with the 1 s shell of nitrogen N through an XPS (X-ray Photoemission Spectroscopy) using a Mg—Kα ray source as shown in FIG. 4. Thenitrogen atom in Ti—O—N of the present embodiment shows a peak in thevicinity of 396-397 eV which is associated with the Ti—N bond.

As described, from the measurement results of the X ray diffraction ofthe Ti—O—N photocatalyst and XPS, it is clear that a chemical bond ispresent between Ti atom and N atom in the Ti—O—N having ananatase+rutile crystal structure.

In general, nitrogen atom may sometimes be mixed, during themanufacturing processes, to powders and films that are commerciallyavailable as titanium oxide for photocatalysts. However, as shown inFIG. 4, in these nitrogen atoms, a peak appears in the vicinity of 400eV. In other words, because the nitrogen atoms that are mixed to theconventional titanium oxide form an organic compound or a nitro group,Ti—N bond is not observed. In this manner, nitrogen which is present inthe titanium oxide and which is mixed during the manufacturing processesor modified on the surface during post-processing cannot affect theelectronic structure of titanium oxide because the chemicalcharacteristics are different.

In addition, in the present embodiment, a charge separation materialsuch as Pt is partially deposited on the surface of the Ti—O—N film 12.

In this manner, by partially depositing onto the surface of thephotocatalytic material, recombination of electrons and holes generatedby the photocatalytic reaction can be prevented, and it is possible toproduce a more efficient photocatalysis reaction.

As the charge separation material to be partially deposited, forexample, a metal element such as Ni, Cu, Ru, Rh, Pd, Ag, Pt, Ir, Au, Re,Os, and Nb or an oxide such as RuO_(x) (for example, RuO₂), NiO_(x) (forexample, NiO), SnO_(x) (for example, SnO₂), Al_(x)O_(y) (for example,Al₂O₃), ZnO_(x) (for example, ZnO), and SiO_(x) (for example, SiO₂) maybe used.

(Embodiment 2)

FIGS. 5A and 5B show structures according to a second embodiment of thepresent invention. In FIG. 5A, a Ti—O—N film 12 is formed on a substrate10, a TiO₂ film 16 is formed on the Ti—O—N film 12, and a chargeseparation material 14 is partially deposited on the surface of the TiO₂film. As the charge separation material 14, a metal element such as Ni,Cu, Ru, Rh, Pd, Ag, Pt, Ir, Au, Re, Os, and Nb, or an oxide such asRuO_(x) (for example, RuO₂), NiO_(x) (for example, NiO), SnO_(x) (forexample, SnO₂), Al_(x)O_(y) (for example, Al₂O₃), ZnO_(x) (for example,ZnO), and SiO_(x) (for example, SiO₂) may be used.

In FIG. 5A, a layered structure consisting of two layers is employed,but the boundary between the layers becomes less distinct through thethermal treatment or the like, resulting in a structure in which Ngradually decreases towards the front surface. In other words, aTiO₂/Ti—O—N film having a graded composition is formed in which thenumber of N atoms decreases towards the front surface, and, at thefront-most plane, TiO₂ is exposed. It is also possible to maintain asharp interface between the Ti—O—N and TiO₂ films.

The construction method of graded composition is not limited to thermaltreatment after the formation of layers of Ti—O—N and TiO₂ films, and agraded composition may also be obtained by changing the gas compositionof the atmosphere based on the deposition state of the films. Morespecifically, by gradually reducing the partial pressure of N₂ in theatmosphere, TiO₂ may be formed on the front surface side.

With such a structure, visible light is absorbed and electrons and holesare produced in the Ti—O—N region (Ti—O—N film 12) near the substrate10. The electrons and holes are supplied to TiO₂ (TiO₂ film 16) in thefront surface of the film. At the front surface, a photocatalytic effectis realized by the TiO₂ film 16.

The metal element which is a charge separation material as describedabove captures the electrons and the oxide captures the holes.Therefore, recombination of electrons and holes produced by thephotocatalytic reaction is prevented and photocatalytic reaction canmore efficiently be generated.

It is also preferable that the TiO₂/Ti—O—N photocatalyst having a gradedcomposition be in a particulate form as shown in FIG. 5B, having aninner Ti—O—N portion 22 and an outer TiO₂ portion 24 with islands of acharge separation material 14 provided on the surface.

In the above description, the photocatalytic material, Ti—O—N, and TiO₂are formed as thin films. The present embodiment, however, can also beapplied to a structure in which a base material constructed by partiallydepositing a charge separation material on the surface of a particulateTiO₂/Ti—O—N is mixed into a binder material for painting.

Moreover, it is possible to create a photocatalytic material using theabove described method as a base method in conjunction with variousparticle creation methods, sol-gel method, chemical reaction methods, orthe like.

(Embodiment 3)

In a third embodiment of the present invention, a Ti—O—S film 18 isemployed in place of the Ti—O—N film 12 as described above. The basicstructure is similar to that of the first embodiment with a notabledifference being that N is replaced by S.

The method for manufacturing such a structure is as follows. First,sputtering is performed using Ti, TiO₂, or TiS (titanium sulfide) as atarget in a SO₂+O₂+inert gas (for example, Ar) to form a Ti—O—S film.Then, thermal treatment (for example, at 550° C. for 2 hours) is appliedto complete the structure. It is also possible to form the Ti—O—S filmonto which a charge separation material is partially deposited throughother manufacturing methods. It is also possible to create the structureas a particle. It is also possible to use CS₂ or H₂S in place of the SO₂gas.

On the surface of the Ti—O—S film 18, a metal element such as Ni, Cu,Ru, Rh, Pd, Ag, Pt, Ir, Au, Re, Os, and Nb, or an oxide such as RuO_(x)(for example, RuO₂), NiO_(x) (for example, NiO), SnO_(x) (for example,SnO₂), Al_(x)O_(y) (for example, Al₂O₃), ZnO_(x) (for example, ZnO), andSiO_(x) (for example, SiO₂) is partially deposited as a chargeseparation material 14.

Similar to Ti—O—N, Ti—O—S is a semiconductor which absorbs visible lightand generates electrons and holes, and which functions as aphotocatalyst with visible light as the operating light. Thephotocatalytic function is augmented by the charge separation material.Therefore, similar to the first embodiment, the photocatalyst of thethird embodiment with a charge separation material partially depositedon the surface of Ti—O—S film realizes similar photocatalytic functionwith the visible light as the operational light.

It is also preferable to use a structure in which the Ti—O—N film 12 ofthe second embodiment is replaced with a Ti—O—S film 18. In this case,TiO₂ film can be formed on the Ti—O—S film 18 in a manner similar tothat in the second embodiment. With such a structure, similar to thesecond embodiment, effective photocatalytic function can be realizedwith the visible light as the operational light.

As described, Ti—O—N and Ti—O—S can be created easily and at low costusing a method such as, for example, introducing nitrogen gas or sulfurdioxide gas to the atmosphere during sputtering. In this manner, it ispossible to realize photocatalytic function responsive to not onlyultraviolet light, but also visible light as the operating light. Inaddition, by placing a charge separation material on the surface of sucha photocatalytic material, it is possible to prevent recombination ofthe produced electrons and holes, and thereby increase photocatalyticfunctionality and effectiveness.

(Embodiment 4)

FIG. 7 is a diagram showing a structure according to a fourth embodimentof the present invention. A film 20 of a photocatalytic material isformed on a substrate 10. Various materials such as SiO₂, glass, andceramic may be chosen for the substrate 10 as suits the intendedapplication.

The photocatalytic material 20 is formed by doping a nitrogen atom (N)or a sulfur atom (S) into a crystal of an oxide of a metal M1 chosenfrom among titanium (Ti), zinc (Zn), and tin (Sn) and doping at leastone of the metal elements M2 chosen from among vanadium (v), chromium(Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu),zinc (Zn), ruthenium (Ru), rhodium (Rd), rhenium (Re), osmium (Os),palladium (Pd), platinum (Pt), iridium (Ir), niobium (Nb), andmolybdenum (Mo).

The results of the ion injection as described above suggest that it isimportant to introduce a metal dopant to the Ti site without cohesion.In the present invention, because of the simultaneous introduction of ametal element M2 and N or S, a photocatalyst which can operate not onlyunder ultraviolet light, but also under visible light can be realized.

In parallel to experiments performed by the present inventors, thepresent inventors evaluated, using a full-potentiallinearized-augmented-plane-wave (FLAPW) calculation method, the electronstates of Ti—O—X based semiconductor photocatalysts in which the O siteis substituted by another element (=B, C, N, F, P, S) and electronstates of Ti—M2—O based semiconductor photocatalysts in which the Tisite is substituted by a metal element M2 (=V, Cr, Mn, Fe, Co, Ni, Cu,Zn, Nb, Mo, Ru, Rh, Pt, Re, Os, Pd, Ir).

FIG. 8 shows a diagram of the density of states based on the calculationresult of the first principle calculation (FLAPW).

The unit cell used in the calculation was a cell in which the Ti sitewas substituted by a metal element M2 (=V, Cr, Mn, Fe, Co, Ni, Cu, Zn,Nb, Mo, Ru, Rh, Pt, Re, Os, Pd, Ir). In FIG. 8, CBM indicates theconduction band minimum and VBM indicates the valence band maximum. Eachrectangle indicates a newly formed optical absorption band. As shown inthe results, a new light absorption band is produced in the band gap ofTiO₂ as a result of the metal substitution. Therefore, absorption ofvisible light by these metal elements can be expected.

FIG. 9 shows results of computation of density states obtained when theO site is substituted by another element (=B, C, N, F, P, S).

As is apparent from these results, an absorption band is produced in thevicinity of the valence band maximum (VBM) of TiO2 because of thesubstitution by N and S. Therefore, absorption of visible light by thesesubstituted elements can be expected.

In addition, the electron states of Ti—O—M2—X (X═N, S) basedsemiconductors in which both elements were doped was also calculated. Asa result, it has been found that the structure is more stable withrespect to energy when the M2 and N or S are at adjacent sites than whenthe M2 and N or S are separated. Therefore, it can be expected thatdeficiencies due to doping such as, for example, increase in the numberof recombination centers, decrease in the carrier mobility, andsolubility limit of M2 and N or S can be reduced because of the stablebonding state of M2 and N or S as described above.

In fact, in the density of states introduced to the band gap of TiO₂ bysimultaneous substitution of a metal element M2 such as Ni, Cu, Pd, Pt,etc. and N, because the energy levels are nearby, a strong hybridizedstate is formed by interaction. It has been confirmed that, because ofthis, the localized density of states in the band gap by M2 is alteredby the simultaneous substitution to be delocalized one. Therefore, theeffects of the potential change caused by the simultaneous substitutionare small compared to cases of substitution of just M2 or X alone, andformation of recombination centers is unlikely.

The calculation results described above are reflected in models in whichthe Ti site is substituted by M2 and oxygen site is substituted by X,but the structure of the present invention is not limited to theexamples described above. In other words, the present inventionencompasses any titanium compound in which a nitrogen atom or a sulfuratom substitute for a portion of the oxygen site of titanium oxidecrystal, is doped at an interstitial site of lattices of titanium oxidecrystals, or is placed at the grain boundary of polycrystalline assemblyof titanium oxide crystals, and at least one of vanadium, chromium,manganese, iron, cobalt, nickel, copper, zinc, ruthenium, rhodium,rhenium, osmium, palladium, platinum, iridium, niobium, and molybdenumsubstitutes a portion of the metal site, is doped at an interstitialsite of lattices of titanium oxide crystals, or is placed at the grainboundary of the polycrystalline assembly of titanium oxide crystals canalter the electron states of the titanium oxide, and can enableabsorption of visible light.

Here, it is preferable that the N or S has a chemical bond with M1 inthe oxide crystal of M1. In other words, by including such a chemicalbond of M1-N, absorption of visible light is enabled. In addition, bydoping a metal element M2, further absorption of visible light can beenabled.

Such a photocatalytic material film 20 can be obtained by, for example,sputtering under a nitrogen atmosphere using an oxide of a metal M1 anda metal M2 as targets.

Next, as a specific example of the present embodiment, a photocatalyticmaterial having a Ti—Cr—O—N structure in which N and Cr are doped to aTiO₂ crystal will be described.

In this example, the photocatalytic material was produced through RFmagnetron sputtering. As the targets, a TiO₂ target and a Cr target eachhaving a diameter of 4 inches were used. With these targets, sputteringwas performed in a 40% N₂—Ar atmosphere and 0.5 Pa and thermal treatmentwas applied at 550° C. in a N₂ atmosphere for 90 minutes forcrystallization, and a Ti—Cr—O—N film was produced. The input power were600W ★2 for TiO₂ and the input power for Cr was varied in a range of10-40 W.

An additional TiO₂ film was formed for comparison purposes. For thisstructure, the target was sputtered in a 20% O₂—Ar atmosphere, andthermally treatment was applied at 450° C. in an O₂ atmosphere for 90minutes for crystallization.

The crystallinity of the Ti—Cr—O—N film was observed through X raydiffraction, and both diffraction lines of anatase TiO₂ and rutile TiO₂have been observed. No diffraction lines were observed which areassociated with a Cr compound or TiN.

FIG. 10 shows the light absorption spectrum of the film. In theTi—Cr—O—N film, the absorption edge is shifted towards a longerwavelength compared to the TiO₂ film. This is because one or more newlevels are formed within the band gap of TiO₂ by the Cr and N dopingsuch that the effective band gap is narrower.

The photocatalytic function of the film was evaluated throughdecomposition performance of methylene blue. This evaluation wasperformed by measuring the decomposition performance of methylene blueapplied onto the surface of the Ti—Cr—O—N film as changes in absorbance(ΔABS) of the film at a wavelength of 600 nm. An 500 W Xe lamp was usedas the irradiation light source. Tests were performed wherein lightincluding an ultraviolet component having a wavelength λ of 200 nm orgreater was irradiated and wherein visible light having a wavelength λof 380 nm or greater was irradiated by using an optical filter limit thewavelengths of the irradiated light.

FIG. 11 shows the experimental results. As shown, with Ti—O—N, highercatalytic activity can be obtained in the range from ultraviolet throughvisible light (λ≧200 nm), compared to TiO₂. In addition, it can be seenthat the catalytic activity is further improved in Ti—Cr—O—N. This isbecause the catalytic activity in response to visible light having λ of380 nm or greater was enhanced by the Cr and N doping. It can be seenthat this result reflects the light absorption spectrum characteristicshown in FIG. 10.

As shown in FIG. 11, the Ti—Cr—O—N photocatalytic material exhibitsphotocatalytic function when irradiated with visible light. In otherwords, with a Ti—Cr—O—N photocatalytic material, photocatalytic functionnot only in response ultraviolet light, but also in response to visiblelight, can be realized, and improvements in hydrophilicity (reduction inthe contact angle of water) and decomposition of organic matters can beobtained. Therefore, Ti—O—N can not only operate with visible light asthe operating light, but, as a result, exhibits significantly improvedphotocatalytic function by using light in the range from ultra violetthrough visible light.

As described above, from the measured results of X ray diffraction andXPS of the Ti—O—N photocatalyst according to the present invention, itis clear that a chemical bond is present between Ti atom and N atomwithin Ti—O—N which has a crystal structure of a combination of anataseand rutile.

In general, nitrogen atom may sometimes be mixed into powders and filmsthat are commercially available as titanium oxide for photocatalysisduring their manufacturing processes. However, as shown in FIG. 4, inthese nitrogen atoms, a peak appears in the vicinity of 400 eV. In otherwords, because the nitrogen atoms that are mixed into the conventionaltitanium oxide form an organic compound or a nitro group, Ti—N bondingis not observed. Therefore, nitrogen which is present in the titaniumoxide and which is mixed during the manufacturing processes or modifiedon the surface during post-processing have different chemicalcharacteristics.

In the present embodiment, by further doping Cr or the like, theelectronic states can be further changed so that the efficiency for theuse of the visible light can be improved.

Next, results of methylene blue decomposition experiments for Ti—(Co,Cu, Ni)—O—N photocatalysts in which other metals are doped along with Nare shown in Table 1. The performance of photocatalysts for irradiationlight having a wavelength λ of 380 nm or greater depends on the amountof doping. The results shown in Table 1 are the maximum values for eachsystem. Although the catalytic activity slightly varies depending on thetype of dopant, a catalytic activity of one order of magnitude higherthan that for TiO₂ can be achieved by doping each of these metals and N.

The compositional ratios in these structures were Ti₂₆Co₁O₇₁N₂ andTi₂₇Ni₁O₇₀N₂. Both of these compositional ratios are heavy in oxygen,but the compositional ratio is not limited to such condition and areductive compositional ratio such as, for example, Ti₃₃Ni₂O₆₃N₂ mayalso be used. In the bonding energy spectrum of 1 s shell of N in an XPSanalysis using Mg—Kα ray, a peak associated with the bonding between anN atom and a metal atom was observed in the vicinity of 396-397 eV.

This range of compositional ratios of oxygen is similar when S doping isused in place of N.

TABLE 1 AMOUNT OF MB DECOMPOSITION PHOTOCATALYST (IN UNITS OF VALUE FORTiO2) TiO2 1.0 Ti-O-N 24.1 Ti-Cr-O-N 30.6 Ti-Co-O-N 29.2 Ti-Cu-O-N 24.2Ti-Ni-O-N 29.8

As a metal element M2, in addition to Cr, vanadium (V),manganese (Mn),iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), ruthenium(Ru), rhodium (Rh), rhenium (Re), osmium (Os), palladium (Pd), platinum(Pt), iridium (Ir), niobium (Nb), molybdenum (Mo) or the like can beused. The metal element to be used is not limited to use of just one ofthese, and two or more of these metal elements can be used incombination.

In the present invention, at least one metal element M2 of palladium,chromium, manganese, iron, cobalt, nickel, copper, zinc, ruthenium,rhodium, rhenium, osmium, platinum, palladium, iridium, niobium, andmolybdenum substitutes a portion of M1 sites of an oxide crystal, isdoped at an interstitial site of the oxide crystal lattices, or isplaced in the grain boundary of polycrystalline assembly of oxidecrystals.

In addition, although in the above example Ti—O was used as a basematerial, a photocatalyst exhibiting a similar photocatalytic functioncan also be obtained by doping at least one metal M2 and nitrogen orsulfur (or N+S) to an oxide semiconductor such as, for example, ZnO,SrTiO₃, SnO₂, WO₃, ZrO₂, Nb₂O₅, Fe₂O₃, Cu₂O, FeTiO₃.

In the above description, an example was described wherein a thin filmwas deposited by sputtering. However, the photocatalytic characteristicis intrinsic to the material, and similar characteristic can be obtainedin a thin film formed through evaporation, a thin film formed throughsol-gel method, or in the form of a fine particle.

(Embodiment 5)

FIGS. 12A and 12B show a fifth embodiment of the present invention. InFIG. 12A, a Ti—Cr—O—N film 22 is formed on a substrate 10 and a TiO₂film 24 is formed thereon.

In FIGS. 12A and 12B, a layered structure of two layers is employed,but, through processes such as thermal treatment, the interlayerboundary between the two layers becomes less distinct, and a structurein which the amounts of N and Cr decrease towards the front surface isobtained. More specifically, a TiO₂/Ti—Cr—O—N film is formed which has agraded composition in which the amounts of N and Cr atoms become smalleras the front surface is approached and TiO₂ is exposed at the front-mostsurface. Alternatively, a sharp interface between the Ti—Cr—O—N film 22and the TiO₂ film 24 can also be maintained.

In addition to thermal treatment after the formation of layers ofTi—Cr—O—N film 24 and the TiO₂ film 24, a graded composition may also beobtained by varying the gas composition of the atmosphere and conditionsfor sputtering Cr based on the deposition state of the film. Morespecifically, by gradually decreasing the partial pressure of N₂ in theatmosphere and the amount of Cr sputtering, it is possible to obtainTiO₂ at the front surface.

In such a structure, visible light is absorbed by the Ti—Cr—O—N region(Ti—Cr—O—N film 22) closer to the substrate 10 and electrons and holesare formed. These electrons and holes are supplied to TiO₂ (TiO₂ film24) at the front surface of the film. At the front surface, thephotocatalytic function is realized by the TiO₂ film 24.

A TiO₂ film is stable and particularly superior in hydrophilicity, andreceives the electrons and holes from the inside so that preferablefunctions such as hydrophilic disinfection function and contaminationprevention are achieved.

It is also preferable to form the TiO₂/Ti—Cr—O—N photocatalyst in aparticulate form as shown in FIG. 12B in which a Ti—Cr—O—N portion 22 isprovided inside and a TiO₂ portion 24 is provided on the outside. It ispreferable that such particulate photocatalyst be mixed to a binder forpaint and employed as a paint.

In addition, various M1—M2—O—N can be used in place of Ti—Cr—O—N of theTi—Cr—O—N film 22 in this embodiment. Moreover, it is also preferable touse Ti—O—N in place of TiO₂ in the TiO₂ film 24. Ti—O—N is stable and issuperior in functions such as decomposition of organic substances. Thus,by using Ti—O—N as the outside material, a characteristic photocatalyticfunction can be achieved.

Moreover, by partially depositing, on the surface of the photocatalyticmaterial according to the present invention, at least one of a metalelement such as Pt, Pd, and Ni, or an oxide such as ruthenium oxide, tinoxide, zinc oxide, aluminum oxide, and nickel oxide, it is possible toform a photocatalyst having an enhanced activity.

As described, M1—M2—O—N (or S) can be created easily and at low costthrough methods such as, for example, sputtering the oxide of M1 whileintroducing nitrogen gas, sulfur dioxide gas, hydrogen sulfide gas, orcarbon sulfide gas to the atmosphere, and then sputtering M2. With sucha structure, a photocatalytic function can be realized having thevisible light as the operational light. By introducing M2, one or morenew levels is created in the band gap, and it is possible to furthershift the light absorption characteristic towards a longer wavelength.

Industrial Applicability

A photocatalyst according to the present invention can be placed on thesurface or the like of various products for decomposition of organicmatters and anti-fogging.

What is claimed is:
 1. A photocatalyst comprising: a titanium compoundin which a nitrogen atom or a sulfur atom is substituted for a portionof oxygen sites of a titanium oxide crystal, in which the same is dopedat an interstitial site of the crystal lattices of titanium oxide, or,in which the same is placed at a grain boundary of a polycrystallinebody of the titanium oxide crystals; and a charge separation materialpartially supported on the surface of the titanium compound.
 2. Aphotocatalyst comprising: a titanium compound in which a nitrogen atomor a sulfur atom is substituted for a portion of oxygen sites of atitanium oxide crystal, in which the same is doped at an interstitialsite of the crystal lattices of the titanium oxide, or, in which thesame is placed at a grain boundary of the polycrystalline body of thetitanium oxide crystals, and in which at least one of vanadium,chromium, manganese, iron, cobalt, nickel, copper, zinc, ruthenium,rhodium, rhenium, osmium, palladium, platinum, and iridium issubstituted for a metal site of the titanium oxide crystal, in which thesame is doped at an interstitial site of the crystal lattices oftitanium oxide, or, in which the same is placed at a grain boundary ofthe polycrystalline body of titanium oxide crystals; and a chargeseparation material partially supported on the surface of the titaniumcompound.
 3. A photocatalyst according to claim 1 or 2, wherein thecharge separation material is at least one metal selected from the groupconsisting of Pt, Pd, and Ni.
 4. A photocatalyst according to claim 1 or2, wherein the charge separation material is at least one oxide selectedfrom the group consisting of RuO₂, NiO₂, SnO₂, Al₂O₃, ZnO, and SiO₂. 5.A photocatalyst comprising: a titanium compound in which a nitrogen atomis substituted for a portion of oxygen sites of titanium oxide crystal,in which the same is doped at an interstitial site of the crystallattices of titanium oxide, or, in which the same is placed at a grainboundary of the polycrystalline body of titanium oxide crystals; and atleast one of Pt, Pd, Ni, RuO₂, NiO₂, SnO₂, Al₂O₃, ZnO, and SiO₂ ispartially supported on the surface of the titanium compound.
 6. Aphotocatalyst comprising: an inner material comprising a titaniumcompound in which a nitrogen atom or a sulfur atom is substituted for aportion of oxygen sites of a titanium oxide crystal, in which the sameis doped at an interstitial site of the crystal lattices of the titaniumoxide crystal, or, in which the same is placed at a grain boundary ofpolycrystalline body of the titanium oxide crystal; a titanium oxidelayer formed on the surface of the inner material; and a chargeseparation material partially supported on the surface of the titaniumoxide layer.
 7. A photocatalyst comprising: an inner material comprisinga titanium compound in which a nitrogen atom or a sulfur atom issubstituted for a portion of the oxygen sites of 2 titanium oxidecrystal, in which the same is doped at an interstitial site of crystallattices of the titanium oxide, or, in which the same is placed at agrain boundary of the polycrystalline body of titanium oxide crystal,and in which at least one of vanadium, chromium, manganese, iron,cobalt, nickel, copper, zinc, ruthenium, rhodium, rhenium, osmium,palladium, platinum, and iridium substitutes for a portion of the metalsites of the titanium oxide crystal, in which the same is doped at aninterstitial site of the crystal lattices of titanium oxide, or, inwhich the same is placed at a grain boundary of a polycrystalline bodyof the titanium oxide crystal; a titanium oxide layer formed on thesurface of the inner material; and a charge separation material ispartially supported on the titanium oxide layer.
 8. A photocatalystaccording to claim 6 or 7, wherein the compositional ratios in thetitanium oxide layer and in the inner material gradually change from thefront surface towards the inside.
 9. A photocatalytic materialcomprising: an oxide crystal of a metal element M1 having aphotocatalytic function in which a nitrogen atom or a sulfur atom issubstituted for a portion of oxygen sites, in which the same is doped atan interstitial site of the crystal lattices of an oxide, or, in whichthe same is placed at a grain boundary of a polycrystalline body of theoxide crystal; and at least one metal element M2 of vanadium, chromium,manganese, iron, cobalt, nickel, copper, zinc, ruthenium, rhodium,rhenium, osmium, palladium, platinum, iridium, niobium, and molybdenumwhich substitutes for a portion of the M1 sites of the oxide crystal, inwhich the same is doped at an interstitial site of the crystal latticesof the oxide, or, in which the same is placed at a grain boundary of apolycrystalline body of the oxide crystal.
 10. A photocatalytic materialaccording to claim 9, wherein the metal element M1 is any one oftitanium, zinc, and tin.
 11. A photocatalyst comprising: an innermaterial comprising a photocatalytic material according to either claim9 or 10; and an outer material formed on the surface of the innermaterial, the outer material comprising a titanium oxide layer or atitanium oxide layer containing nitrogen or sulfur.
 12. A photocatalystaccording to claim 11, wherein the compositional ratios in the outermaterial and in the inner material gradually change according to thedistance from the front surface.